Abstract Lymph nodes (LNs) are the quintessential organs of immunity. Our understanding of how LNs control alloimmune responses has evolved significantly with recent advances which highlight the function of specific cellular and stromal components of the LN. Presentation of donor alloantigen to recipient T cells in the LN is fundamental to the priming of allo-reactive T cells and subsequent allograft rejection. A recently recognized, new dimension to this pervasive concept is that the LN is also critically important for transplant tolerance. These multifaceted functions reflect the status of LNs as extremely specialized organs with unique microvasculature, and a stromal compartment that is molded and regulated by resident mesenchymal cells known as fibroblastic reticular cells (FRCs). The overall hypothesis of this project is that sustained activation of FRCs in the draining LN (DLN) following transplantation results in their transformation to scar-forming pro- inflammatory myofibroblasts, which further promote alloimmunity. Our studies will focus mechanistically on the importance of the LIGHT/HVEM signaling pathway to the differentiation of FRCs into proinflammatory myofibroblasts in the LN following transplantation. Our corollary hypothesis is that restoration of the native function of FRCs through targeted drug delivery to LNs will enhance their immunoregulatory function and promote tolerance. We are proposing three AIMS as follows: In AIM 1, we will examine the role of the LIGHT/HVEM pathway in regulating the function of FRCs, controlling extracellular matrix (ECM) accumulation, and mediating transplant immunity. Using global LIGHT and HVEM knockout mice, and mice with conditional knockout of HVEM on FRCs, we will gain mechanistic insights into how the LIGHT/HVEM pathway controls the differentiation of FRCs and transplantation outcomes. In AIM 2, we will study the importance of senescence in determining the balance between the pro-inflammatory and anti-inflammatory properties of FRCs. We will use a number of innovative conditional knockout mice to decipher the mechanisms by which senescent FRCs promote alloimmunity following transplantation. In AIM 3, we will determine whether delivering healthy FRCs and senolytic agents to the DLN will restore its microarchitecture and regulate alloimmunity following transplantation. The data from these studies will lay the groundwork for the first time to develop innovative therapeutic strategies aimed at manipulating the microenvironment within LNs. This provides a unique opportunity to direct the alloimmune response following transplantation towards tolerance. This proposal establishes a multidisciplinary collaborative team to produce novel mechanistic data, which will provide the basis for highly innovative and selective therapeutic strategies for transplantation. Therefore, this proposal can make transformative advances in the field of organ transplantation.